This invention relates generally to rotary machines and more particularly, to methods and apparatus for monitoring rotary machines.
Some known wells, such as oil wells, are formed by drilling a borehole within a natural formation below the surface of the Earth. Such formations may be found below land-based surfaces and/or submerged surfaces. Some known drilling methods use powered rotating equipment to induce torque to a drill pipe that subsequently rotates a drill bit. The rotating drill bit bores into the formation and generates cuttings of the formation to form a drilling well while appropriate fluids that facilitate transporting the cuttings to the surface are circulated within the well. The drill pipe is lowered and raised within the drilling well by a support cable extending from a drawworks drum. When rotating, the drawworks drum extends and retracts the cable to cause the drill pipe to be lowered and raised, respectively. A pre-determined rate and amount of drill bit movement within the drilling well is influenced by a number of variables that include, but are not limited to a hardness of the formations being drilled and/or a need to withdraw the drill pipe from the well to replace the drill bit. Facilitation of the drilling activities is at least partially attained by determining a depth of the drill bit within the well. The drill bit depth is typically attained by monitoring the length of drill pipe inserted into the drilling well, as well as the rate and direction of movement of the drill pipe.
To facilitate determining such drill bit depth, some known drilling assemblies include drill bit measurement devices including encoders that measure the rotation of the drawworks drum. The encoders transmit data to a monitoring system that correlates rotation of the drawworks drum to a drill pipe depth. However, because some known encoders require an external power source to supply a power level above 0.25 watts and voltages above 24 volts DC, such encoders may not be suitable for use in areas wherein an ignitable environment may exist.